Water Structure & Function

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Function of Water

Water is a major component of cells and makes up 60-70% of the human body. Life evolved in an environment where water was abundant. It has several properties that are important in biology.

Importance of water

Water is one of the most useful molecules for life. Its uses include:
- As a reactant in cells (e.g. photosynthesis, hydrolysis).
- Provides structural support in cells.
- Keeps organisms cool to maintain an optimum body temperature.

Properties of water

Special properties of water are:
- Metabolic importance.
- High heat capacity.
- Heat of vaporization.
- Cohesive properties.
- Useful as a solvent.

The Structure of Water

The structure of a water molecule helps us to understand hydrogen bonding and the function of water.

Contents of a water molecule

Water molecules (H₂O) are made from:
- One oxygen atom.
- Two hydrogen atoms.

Polarity

Water is a polar molecule.
- The oxygen atoms in water are slightly negatively charged.
- The hydrogen atoms in water are slightly positively charged.

Hydrogen bonding

The polarity of water molecules means that a hydrogen atom on one water molecule is attracted to the oxygen atom on another water molecule.
This attraction is called hydrogen bonding.

Useful Properties of Water

Water is a major component of cells and is essential to life as we know it (60–70% of the human body is made up of water). The properties of water that make it such a useful substance are:

High latent heat of vaporisation

The latent heat of vaporisation is the amount of energy needed to change one gram of a liquid substance to a gas.
As liquid water heats up, hydrogen bonding makes it difficult to separate the water molecules from each other. This means that a lot of energy is needed for water to evaporate.
- When water evaporates, energy is used up - this cools the environment where the evaporation is taking place.
- This is why sweating helps with body temperature regulation.

High heat capacity

Specific heat capacity is the amount of heat one kilogram of a substance must absorb or lose to change its temperature by 1^oC.
Water has a high heat capacity so takes a long time to heat and cool.
- The specific heat capacity of water is much larger than sand. This is why land cools faster than the sea.
- Water is used by warm blooded animals to more evenly disperse heat in their bodies.

Good solvent

Water is a good solvent because ions and polar molecules can easily dissolve in it.
Water is a polar molecule. This means that the positive end of the water molecule attracts negative ions and the negative end will attract positive ions.

Good metabolite

Water is used or formed in many metabolic reactions, such as condensation and hydrolysis reactions.
- ATP + H₂O → ADP + P_i + energy
  - A bond is broken and a water molecule is used up and so this is a hydrolysis reaction.
- ADP + P_i + energy → ATP + H₂O
  - A new bond is formed and a water molecule is released and so this is a condensation reaction.

Cohesive properties

The strong attraction between water molecules due to hydrogen bonds is called cohesion.
Cohesion produces surface tension where water meets air.
- This is why water forms droplets when placed on a dry surface rather than being flattened out by gravity.
- Plants use this natural phenomenon to help transport water from their roots to their leaves.